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Everolimus, a proliferation signal inhibitor, prevents allograft rejection in rodent and nonhuman primate models of allotransplantation. It exerts its immunosuppressive effect by inhibiting the proliferation and thus, clonal expansion, of antigen-activated T cells which is driven by T cell-specific interleukins eg, interleukin-2 and interleukin-15. Everolimus inhibits an intracellular signaling pathway that normally leads to cell proliferation when triggered by the binding of these T cell growth factors to their receptors. The blockage of this signal by everolimus causes cells to be arrested at the G1 stage of the cell cycle.
At the molecular level, everolimus forms a complex with the cytoplasmic protein FKBP-12. In the presence of everolimus, the growth factor-stimulated phosphorylation of the p70 S6 kinase is inhibited. Since p70 S6 kinase phosphorylation is under the control of FRAP (also called m-TOR), this finding suggests that the everolimus-FKBP-12 complex binds to and thus, interferes with the function of FRAP. FRAP is a key regulatory protein which governs cell metabolism, growth and proliferation; disabling FRAP function thus explains the cell cycle arrest caused by everolimus.